The accuracy of focus can be problematic for camera/lens systems which employ a different light path between image focusing and recording locations. For example, camera systems such as the single lens reflex (“SLR”) or digital single lens reflex (“DSLR”) cameras typically have three different light paths through which an image can be focused. With both type of cameras, the light path from the primary lens to the image recording device, such as photographic film, or a light sensitive solid state image sensor, differs from the path that light travels to reach the focusing sensor, which may be either an auto-focus system or a manual eyepiece.
As illustrated in the camera and lens system 10 of FIG. 1, when manually focusing the image the majority of the light from target object “O” passes through the primary lens 20 and reflects upward off the vertical focusing mirror 22 to impact the underside of the transparent matte view surface 24. This distance is referred to as light path OA. The top surface of the transparent matte is viewed through an eyepiece 26 by the photographer, who makes adjustments to the primary lens to bring the image on the matte into focus. It is critical that light path OA be substantially equal in length to light path OB, which is the distance that light travels before reaching the image recording device 30 after the mirror 22 pivots upward to take the picture. Otherwise, the light traveling along light path OB and impacting the image recording device 30 will be out-of-focus or focus-shifted. This problem can be present in cameras having film, a charge-coupled device (“CCD”), a CMOS detector, or similar sensor, etc. as the image recording device 30.
In auto-focusing cameras, a fraction of the light from target object “O” can also travel through a semi-transparent window 32 in the vertical focusing mirror 22 to be directed downward by the auto-focus mirror 34 into the auto-focus detector 36. This distance is referred to as light path OC, and like distance OA, must be substantially equivalent in length with OB; if not, any image captured by the image recording device 30 during auto-focus operation may also be out-of-focus or focus-shifted.
An SLR- or DSLR-type camera system is focused either manually through the eye-piece 26 or using the auto-focus detector 36 so that a plane of focus 12 is centered on about a target object O within an image. Furthermore, the camera/lens system 10 can also be set to have a desired depth of field 14 (or in-focus area), which is the distance in front of and in back of the object which is also acceptably in focus. Ideally, the plane of focus 12 selected by the focusing sensor is centered within the depth-of-field 14 in the image reaching the image recorder. However, it is quite possible for small inaccuracies to be present. The inaccuracies can be caused by problems within the camera body, the attached lens, or in a combination. For example, in the camera body there may be positioning and/or alignment problems with the semi-transparent window 32, the auto-focus detector 36, the auto-focus mirror 34, the image recording device 30, the transparent matte 24, and so forth. With auto focusing systems, errors may also be caused by the primary lens motor which is designed to quickly move the relatively-heavy lens to focus the image, resulting in problems both if the lens is moved too far or not far enough. A variety of other potential problems can also occur which shift the depth-of-field 14 forward or backward relative to the desired plane of focus 12.
If the plane of focus 12 is too far forward, for instance, only the portion of the imaged target object that is just behind the plane of focus, as viewed through the focusing sensor, will be in focus in the final image. This is known as back-focusing. Similarly, if the plane of focus is too far backward the resulting picture will be front-focused, and only the portion of the imaged target object which is just in front the plane of focus will appear in focus. If the degree of back- or front-focusing is known or measured, small screws with cams 42, 44 can be turned to adjust the mirrors, thereby equalizing the focusing light paths OA, OC with the image light path OB, and bringing the plane of focus back into alignment so that it is nearly centered at the object plane.
Basic measurement systems have been developed to evaluate the accuracy-of-focus for cameras that are susceptible to these types of focusing problems. Generally, the measurement system involves capturing an image of a scale printed on a piece of paper that has been tilted at a 45 degree angle relative to the plane of the image sensor. If the camera is correctly aligned and focused on a target located in the center of the scale, the demarcations on the scale will be in focus in the center of the image and out-of-focus towards the upper and lower edges of the image. Analyzing the position of the in-focus portion of the image on the scaled piece of paper can give an indication as to the location of the plane of focus and the width of the depth-of-field of the camera/lens system.
These measurement systems suffer from the common difficulty in establishing a repeatable alignment between the measurement plane and the camera, as most cameras must be both aligned square to and centered on the focusing target in order to obtain repeatable and accurate readings. Unfortunately, proper alignment of the camera system to the paper target is easier said than done, and accurately focusing the camera on the focus target in the center of the image, especially with an auto-focus detector, can also be problematic. Furthermore, the fine grid lines, small lettering and fixed perspective of the prior art systems can lead to difficulties in accurately identifying the in-focus and out-of-focus portions of the image. The inability to obtain repeatable measurements of the depth of field 14 relative to the plane of focus 12 can make it difficult to properly adjust or calibrate a camera/lens system. Additionally, sending a camera/lens system to a manufacturer or specialist for adjustment of the depth of field can be problematic if the owner and manufacturer or specialist uses different or inaccurate testing procedures.